Sourcing materials yourself can offer significant savings, as long as you buy the right things at the right price. Although contractors will want to offer competitive quotations, they are unlikely to invest hours finding unique items you want at the best price. Instead, they will most likely use their usual merchant. If you are prepared to do the leg work and find where those special products can be sourced at the best prices, significant savings can be made as a result.
Also, think about sourcing materials second-hand, ideally from other self-builders who may have over-ordered for their projects. You can also look for generic branded products via eBay or Facebook marketplace, but remember to make sure the condition and specification are what you need — otherwise, a great saving can turn into an unnecessary expense.